538 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



OLF. N.F. 



FIG. 5. General arrangement of the neural paths in the ol- 

 factory bulb. Fibers from the receptor cells are collected on 

 the surface of the bulb (OLF .„V.F.) and participate in the for- 

 mation of more deeply situated glomeruli iGLOM') which 

 also receive the dendritic processes of the mitral cells (^W) and 

 the tufted cells (T). Axons of mitral cells are mainly collected 

 into the lateral olfactory tract QLAT. OLF. TR.') and run to 

 the primary olfactory cortex. The finer axons of tufted cells 

 pass into the anterior limb of the anterior commissure, reach- 

 ing the opposite bulb where they synapse with deeply situated 

 granule cells (G). .■\xons of granule cells are directed peripher- 

 ally at least as far as the fields of the mitral and tufted cells. 

 OLF. VEjVT. represents the olfactory ventricle present in 

 lower mammals and continuous with the cerebral ventricular 

 system. 



anterior and dorsal parts of the olfactory mucosa 

 reach the anterior parts of the iaulia, whereas the 

 ventral and posterior reE;ions of the mucosa project 

 to the posterior parts of the bulb (5). Although initial 

 anatomical studies (35) did not support a topographic 

 arrangement, more extensive investigations (31) have 

 confirmed the general arrangement suggested bv 

 Adrian. The upper part of the olfactory epithelium 

 projects mainly to the upper part of the bulb, and 

 lower areas of epithelium to lower regions of the 

 bulb; but anatomical evidence of an anteroposterior 

 organization of the projections is less definite. 



ESSENTIAL PROCESSES INVOLVED IN 

 OLFACTORY STIMULATION 



Characters nf Odorous Substances 



No embracing picture can yet be advanced to 

 categorize all odorous substances, since many are 

 totally unrelated physically and chemically. Hill & 

 Carothers (54) observed a relationship between the 

 number of atoms in certain macrocyclic ring hydro- 



carbon compounds and the nature of their odors. 

 Thus compounds with 13 atoms possess a cedar-like 

 odor, with 14, 15 or 16 atoms a musk-like odor, and 

 with 17 or 18 atoms a civet-like odor. For example, 

 pentadecanolide and decamethylene oxalate both 

 have musky odors. 



-CO 



(CH,)i4 



(CHOi, 



Pentadecanolide 



O 



- O 



I 



CO 



! 



CO 



I 



- o 



Decamethvlene oxalate 



Hill & Carothers suggest that, within wide limits, 

 the number of atoms in the ring, rather than the 

 identity of the reactive groups, is the significant fac- 

 tor. However, many anomalies have limited attempts 

 to extend this hypothesis. Moncrieff (68) has sug- 

 gested that to be odorous a substance must be volatile 

 and soluble in the tissues of the olfactory mucosa, the 

 latter property involving varying degrees of water 

 and fat solubility. The olfactory mucous membrane 

 exhibits considerable powers of adsorbing odorous 

 substances in the freshly isolated state, and this 

 sorptive property may be intimately related to proc- 

 esses of excitation in olfactory receptors (69). Mon- 

 crieff further suggests that the disposition of an ele- 

 ment in the electrochemical series may be correlated 

 with odorous properties, since only seven elements 

 are odorous (fluorine, chlorine, bromine, iodine, 

 oxygen as ozone, phosphorus and arsenic), and six 

 of these occupy the lowest places in the electrochemi- 

 cal series. The disposition of substitution groups in 

 organic compounds is of great significance in deter- 

 mining both strength and quality of odors. Legge 

 (64) has advanced the hypothesis that odorous sub- 

 stances may react with groups on the free surface of 

 protein and lipoprotein films, leading to the rupture 

 of a few bonds in incompletely spread proteins with 

 a consequent enorinous increase in their area. In view 

 of the role played by — S — S — groups in the main- 

 tenance of protein structure, the rupture of bonds 

 induced by odorous substances might explain the 

 high dilutions at which mercaptans can be detected. 



On the basis of records from the olfactory bulb, 

 .\drian (10) has defined four groups of .substances, 

 and in each group has detected one substance capable 

 of evoking a discharge limited to a single one of the 

 units within the range of the electrodes. Acetone 



